Hot air rises. Every child is taught this at school as a statement of fact and with very little proof. It is explained that water vapour rising is the basis of the water cycle in nature and of our weather. Similarly, in school science we are taught that hot water rises and that this is fundamental to the operation of the domestic hot water system. They are both examples of convection currents. Yet there is virtually no other example of their application in the domestic or industrial use of energy.
The old fashioned coal fire depends on convection currents. As the fuel burns, the hot combustion products lose some of their energy into the room but carry most of the heat up the chimney. Fresh air is drawn in underneath the fire. The principles of combustion are everyday knowledge but there is no example of harnessing the energy of the rising hot gases other than directly for heating.
The steam engine was invented and developed over 200 years ago by James Watt and others. It was one of the key technologies that made possible the industrial revolution. For the first time in history heat energy from burning coal could be converted into mechanical energy to drive machines and for transport.
One hundred years later the steam engine was the basis for the generation of electricity. Heat from burning fossil fuels is used to turn water into high pressure steam which drives a turbine. Unfortunately, the latent heat of vaporisation of water cannot be recovered and low pressure steam is the embarrassing and massive waste product. Despite the magnificent scientific breakthroughs of the last century, electricity production at the Millennium still depends on steam power and has an efficiency of only 40% or so.
Motor vehicle engines produce their energy by the explosion of a petrol/air mixture. The principles again are over 100 years old. Despite continuing sophisticated refinements, the overall efficiency is only about 20-25%.
Why is it that we cannot improve on such appalling inefficiency? Despite the brilliance of so many technological achievements and despite the imminence and the almost intractable nature of the problems of global warming, the best that science can achieve in the conversion of heat into mechanical or electrical energy is 20-40%.
If there was an efficient way of converting heat into mechanical energy, world energy demand and carbon dioxide emissions could be halved. If it applied to solar energy and was economic, the problems of global warming could be solved.